Refer to Logistics Engineering and Management by Benjamin S. Blanchard for
a complete discussion of this manual process for calculating spares (see the
References Section of this reference guide).
Step 1: Determine the following variables:
FR = Failure rate
AOR = Annual operating requirement (hours per year that product will
DC = Duty cycle (percent of time that unit is functioning)
N = Number of assemblies or components in the end
Step 2: Multiply the variables in Step 1 together. The value obtained is
located on the horizontal axis of the graph.
Step 3: At the value calculated in Step 1, move up in a vertical line until
the appropriate value for confidence level is intersected. (These values are
the percentages on the vertical axis.) The confidence level is the probability
that all but r items (the number on the curve) will operate properly for the
Step 4: By stocking a number of spares corresponding to the number of
anticipated failures (r), sufficient spares should be on hand to meet
Now for an example:
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Step 1: System X includes 4 receivers (N=4). System X
operates 2400 hours per year (AOR = 2400), and the receiver functions
(actually receives signals) 25 percent of the time (DC= .25).
The receiver fails on the average of once every 1000 hours
or .001 failure per hour (FR=
.001). The customer wants the
product to operate properly at least 90 percent of the time (confidence level
Step 2: FR x AOR x DC x N = .001 x 2400 x.25 x 4 = 2.4
Step 3: The vertical line corresponding to 2.4 intersects the 0.9
confidence level line where curve r is approximately equal to 4.
Step 4: Therefore, initial estimates for spares would be 4. This quantity
should be entered into the LSAR H Data Record Sheet or equivalent.
(Reliability Engineering Handbook. Department of the Navy, (NAVAIR 01-1A32).